Program: GN-2019B-Q-227

Cepheids are the first step in the extragalactic distance calibration, and measurement of their most fundamental parameter - the mass - has important astrophysical and cosmological significance. Cepheid masses are also needed to resolve an apparent discrepancy between evolutionary-track predictions and masses from pulsation calculations. Our "Periastron Campaign" proposes to continue the resolved observations of the visual binary Cepheid Polaris with Gemini Speckle Imaging. The close, faint companion of this brightest and nearest Cepheid was first resolved with the Hubble Space Telescope (HST, Evans et al. 2008). However, its 30-year orbit has now moved the companion, Polaris Ab, close to periastron, and thus too close for the HST to resolve. Gemini speckle imaging with 'Alopeke will continue the observations through this important orbital phase. These measures will significantly improve the orbit determination, and reduce the mass error for the Cepheid ultimately to 2% or less (when combined with the Gaia parallax of the wider companion, Polaris B).